1. Ch. 26 – The Urinary System
The general functions of the major organs of the urinary system are stated here
Fig. 26-1, p. 954

2. Specific functions of the urinary system
1. Excretion = the removal of organic metabolic waste products from body fluids (such as blood)
2. Elimination = the discharge of these waste products to the external environment
3. Regulation of blood plasma volume and solute concentration, by…
Regulating blood volume and pressure, by…
Adjusting the volume of water in the urine
Releasing erythropoietin (EPO)
Releasing renin
Regulating plasma concentrations of Na+, K+, Cl-, Ca2+, etc.
Helping to regulate blood pH, by…
Excreting or conserving H+ and HCO3-
Conserving valuable nutrients
Assisting the liver in detoxifying poisons and deaminating amino acids

3. FYI (for lecture) – renal gross anatomy is covered in lab
Fig. 26-3, p. 956

4. FYI (for lecture) – renal gross anatomy is covered in lab
Fig. 26-2, p. 955

5. FYI (for lecture) – renal gross anatomy is covered in lab
Fig. 26-4, p. 956

6. FYI (for lecture) – renal gross anatomy is covered in lab
Fig. 26-5, p. 958

7. Nephrons = the microscopic functional units of the kidneys
Ca2+ reabsorbtion is variable due to PTH and calcitriol
Na+ reabsorption is variable due to aldosterone
H2O reabsorption is variable due to ADH
= the microscopic functional units of the kidneys
Each consists of a renal corpuscle and a renal tubule
Function: form urine via filtration of the blood, reabsorption of water and some solutes, and secretion of other solutes
There are over 1 million nephrons per kidney!
Fig. 26-6, p. 959

8. Cortical nephrons
Make up ~ 85% of all nephrons; their renal corpuscles are located in the outer cortex
The efferent arteriole sends blood to peritubular capillaries that surround the entire renal tubule
Have short nephron loops (loops of Henle) that are mainly in the cortex and superficial region of the medulla
Fig. 26-7, p. 961

9. Juxtamedullary nephrons
Make up ~ 15% of all nephrons; their renal corpuscles are located deep in the cortex
The efferent arteriole sends blood to both peritubular capillaries and the vasa recta
Have long nephron loops which penetrate deep into the medulla and are surrounded by the vasa recta
The ascending and descending limbs of the nephron loop have different permeability characteristics, allowing the production of concentrated urine (more on this later)
Fig. 26-7, p. 961

10. The renal corpuscle
= the glomerular (Bowman’s) capsule + the glomerulus
The glomerular capsule is made up of an outer parietal layer of simple squamous cells called the capsular epithelium and an inner visceral layer of simple squamous cells that cover the glomerulus called the visceral epithelium (the individual cells of the visceral epithelium are called podocytes), with the capsular (Bowman’s) space in between
The glomerulus is a bed of fenestrated capillaries
Fig. 26-8, p. 962

11. The glomerular filtration membrane
Consists of 3 layers:
1. The glomerular capillary endothelium is fenestrated (i.e., the capillary endothelial cells have fenestrations, or pores)
But fenestrations do not let blood cells or platelets pass into the filtrate
2. The dense layer (part of the basement membrane) is between the glomerular endothelium and the visceral epithelium (podocytes)
This prevents the filtration of large plasma proteins
3. The visceral epi thelium covers the glomerulus
It is a simple squamous epi thelium made up of cells called podocytes
Podocytes have pedicels (“little feet”), between which are filtration slits
Fig a,
p. 968
Fig. 26-8b, p. 962

12. Renal physiology: urine formation
Fig. 26-9, p. 965

13. Renal physiology: urine formation
Urine is formed by 3 distinct processes:
1. Filtration = the production of filtrate (“pre-urine”) from blood plasma
Essentially, anything except blood cells, platelets, and most plasma proteins can be filtered (but may not be—see “secretion” below)
It occurs due to blood pressure within the glomerulus of the renal corpuscle
2. Reabsorption = the removal of “wanted” substances from the filtrate (which is now called tubular fluid)
It occurs via diffusion or active transport along the renal tubule and collecting duct
The reabsorbed substances will ultimately be returned to the blood
3. Secretion = “unwanted” substances that did not get filtered from the blood originally are added to the tubular fluid
E.g. metabolic wastes like urea, creatinine, and uric acid
It also occurs via diffusion or active transport along the renal tubule and collecting duct

14. How efficient is urine formation?
Don’t memorize the numbers; just note that the kidneys are good at keeping nutrients and getting rid of wastes
Table 26-2, p. 964

15. Glomerular filtration
Fig , p. 968
Glomerular filtration
Essentially, the hydrostatic (blood) pressure within the glomerulus (= the GHP) forces water and small solutes through the filtration membrane into the capsular space
Net filtration pressure = NFP = (GHP – CsHP) – BCOP
So NFP = (50 mm Hg – 15 mm Hg) – 25 mm Hg = mm Hg

16. The glomerular filtration rate (GFR)
= the volume of filtrate produced by both kidneys in one minute
The kidneys have a HUGE filtration surface area (6 m2/64 ft2 per kidney), so…
Average GFR = 125 mL/minute = 180 L/day!!!
But 99% of the filtrate is reabsorbed
Control of the GFR occurs on 3 interacting levels:
1. Autoregulation (local)
If autoregulation is insufficient to restore normal GFR, then…
2. Hormonal regulation
Via the renin-angiotensin system and natriuretic peptides
3. Autonomic (nervous) regulation
Mostly by the sympathetic division of the ANS (only in a crisis situation)

17. Autoregulation of the GFR
Maintains the GFR when local BP and blood flow through the glomerulus change
1. ↓ GHP or blood flow causes:
Dilation of afferent arterioles
Constriction of efferent arterioles
2. ↑ GHP or blood flow causes:
Constriction of afferent arterioles

18. Hormonal regulation of the GFR
Natriuretic peptides (ANP and BNP)
Are released when ↑ systemic BP and blood volume stretch the walls of the heart
Some effects (there are more—see Ch. 18 or 21) include:
Dilation of the afferent arterioles → ↑ GHP → ↑ GFR → ↑ urine output → ↓ systemic BP and blood volume
Constriction of the efferent arterioles, with the same effect as above
Renin
Is released by the juxtaglomerular complex (JGC) when:
It detects ↓ GHP
It’s stimulated by the sympathetic division of the ANS
Its macula densa cells detect ↓ [osmotic] in the tubular fluid within the DCT
Effect: it ultimately activates angiotensin II (see the next slide)…
Fig. 26-8a, p. 962

19. Once again, the renin-angiotensin system
Fig , p. 971

20. Autonomic regulation of the GFR
Most renal innervation is by the sympathetic division of the ANS
An acute ↓ in systemic BP or blood volume due to a life-threatening emergency (e.g. a heart attack, anaphylaxis, major blood loss, etc.) → sympathetic activation, causing:
Powerful constriction of the afferent arterioles → ↓ GHP → ↓ GFR
I.e., “this is a crisis situation…I must maintain systemic blood volume and pressure at the expense of kidney perfusion and function!”

21. Reabsorption and secretion at the PCT
Reabsorption of 60-70% of the filtrate occurs here
Reabsorbed substances enter the peritubular fluid (= the interstitial fluid that surrounds the renal tubule), then diffuse into the peritubular capillaries
Limited secretion occurs here
Fig , p. 973
Fig. 26-7c, p. 961

22. Reabsorption and secretion at the PCT
Reabsorption of:
Organic nutrients: vitamins, and 99% of the glucose and amino acids that were filtered
Ions: Na+, K+, HCO3- (CO2), Cl-, urea, and others
Water (which osmotically follows the solutes)
Secretion (a limited amount) of:
H+, NH4+, creatinine, drugs, and toxins
Fig , p. 973

23. Reabsorption at the nephron loop
~ 25% of the water and 20-25% of the Na+ and Cl- present in the original filtrate are reabsorbed via countercurrent multiplication (see the next two slides for more details)…
Fluid in the descending and ascending limbs of the nephron loop run in opposite directions (i.e., countercurrent)
The limbs have different permeability characteristics:
The thin descending limb is permeable to water, but not to Na+ and Cl-
The thick ascending limb is not permeable to water, and pumps Na+ and Cl- out
Fig a, p. 975

24. Countercurrent multiplication at the nephron loop
1. Na+ and Cl- are pumped out of the thick ascending limb of the nephron loop
2. This results in ↑ [osmotic] in the peritubular fluid of the medulla
3. Water is attracted to the Na+ and Cl- in the peritubular fluid, and thus it (water) leaves the thin descending limb by osmosis
4. This results in ↑ [Na+ and Cl-] in the tubular fluid of the descending limb as it moves toward the ascending limb
5. The arrival of highly concentrated tubular fluid in the thick ascending limb allows the Na+/Cl- pumps to work more efficiently, pumping even more Na+ and Cl- out to the peritubular fluid (this is a repeat of step #1, so countercurrent multiplication is a positive feedback loop)
Fig b, p. 975

25. More on countercurrent multiplication
Urea adds even more to the [osmotic] in the peritubular fluid of the deepest parts of the medulla
The overall functions of CCM are to:
Efficiently reabsorb NaCl and water
Create and maintain ↑ [osmotic] in the peritubular fluid of the medulla for water reabsorption at the thin descending limb of the loop and the collecting duct
Fig c, p. 975

26. Reabsorption and secretion at the DCT
Reabsorption of:
Ions: Na+ (variable with aldosterone levels*), Cl-, Ca2+ (variable with PTH and calcitriol levels), and HCO (CO2)
Water (vari able with ADH levels)
Secretion (more than what occurs at the PCT) of:
H+, K (counter transported with Na+) NH4+, creatinine, drugs, and toxins
*Aldosterone stimulates the synthesis of Na+/K pumps and Na channels
Fig ab, p. 978

27. Reabsorption and secretion along the collecting system
Reabsorption of:
Ions: Na+ (variable with aldosterone levels), Cl-, HCO3- (CO2) or H+, and urea (at the papillary ducts only)
Water (variable with ADH levels)
Secretion of:
H+ or HCO3-, and K+ (counter-transported with Na+)
Note: H+ and HCO3- generally move in opposition to one another to help regulate the pH of body fluids
Fig c, p. 979

28. Forming dilute vs. concentrated urine
This is regulated by the absence or presence of ADH
ADH causes water channels (aquaporins) to be inserted into the membrane of the DCTs and collecting ducts, which greatly enhances osmosis
Fig , p. 980

29. The general characteristics of normal urine
Note that normal urine is slightly acidic, mostly water, clear (not cloudy or opaque), sterile, and while we may produce 180 L of filtrate per day, only about 1-2 L of urine is actually formed per day
Table 26-5, p. 981

30. A summary of renal function (part 1)
Fig , p. 982

31. A summary of renal function (part 2)
Fig , p. 983

32. Urine transport, storage, and elimination
FYI (for lecture) – gross anatomy is covered in lab
Urine transport, storage, and elimination
Fig , p. 985

33. Urine transport, storage, and elimination
FYI (for lecture) – gross anatomy is covered in lab
Fig , p. 987

34. Urine transport, storage, and elimination
FYI (for lecture) – histology is covered in lab
Fig , p. 988

35. The micturition reflex and urination
Fig , p. 989
The reflex begins to function when ~ 200 mL is in the bladder
If you don’t voluntarily go (step C3-4), the bladder relaxes and the cycle repeats (with increasing intensity) within an hour
Once the volume reaches ~ 500 mL, the pressure generated by the detrusor muscle may be high enough to force open the sphincters
Typically < 10 mL will remain in the bladder after urination is completed
The micturition reflex and urination

36. Admit it: urine love with your kidneys, aren’t you?